Extracellular protein phosphorylation systems in the regulation of cellular responsiveness

In cells which store ATP within secretory vesicles and release it by exocytosis upon cell stimulation, extracellular protein phosphorylation systems may play an important role in cellular activation and intercellular communication. Secretion of ATP has been well documented in three cell types: neurons, chromaffin cells of the adrenal medulla and platelets. In each of these cell types we have found extracellular protein phosphorylation systems and surface proteins phosphorylated by extracellular ATP. Two types of extracellular protein kinases have been detected: a membrane bound ecto-protein kinase and a soluble, secreted exo-protein kinase. The ecto-protein kinase has properties consistent with involvement in the feedback regulation of cellular activation. The secreted protein kinase can phosphorylate proteins of the extracellular matrix and surface proteins of target cells, and thus may serve as an enzyme with the role of first messenger in cell-cell communication.